Pornography exists for no reason other than sexual stimulation. It might be framed in artistic frameworks, such as a story, and it is always a subset of an artistic genre (film, literature, video game, etc.), but arguing it as an art itself is questionable. All mediums have uses that aren't actually art (i.e. a Terms of Service agreement is not art, nor is a Youtube video about how to tie a tie, etc.), and it's very easy to argue that something made for the sole purpose to stimulate someone sexually does not qualify as art.